The information provided herein and references cited are provided solely to assist the understanding of the reader, and does not constitute an admission that any of the references or information is prior art to the present invention.
Neurodegenerative diseases are disorders characterized by destruction or deterioration of selective neuronal populations. Exemplary neurodegenerative diseases include Alzheimer's disease (AD), Parkinsonian syndromes such as (PD), Huntington's disease (HD), Prion diseases, cerebral amyloid angiopathy (CAA), and mild cognitive impairment (MCI). Neurodegenerative disease is associated with progressive nervous system dysfunction, and often leads to atrophy of affected central or peripheral nervous system structures.
Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is the predominant cause of dementia in people over 65 years of age. Clinical symptoms of the disease begin with subtle short-term memory problems. As the disease progresses, difficulty with memory, language and orientation worsen to the point of interfering with the ability of the person to function independently. Other symptoms, which are variable, include myoclonus and seizures. Duration of AD from the first symptoms of memory loss until death is 10 years on average.
AD is characterized by massive neuronal cell loss in certain brain areas, and by the deposition of proteinaceous material in the brains of AD patients. These deposits contain neurofibrillary tangles and β-amyloid plaques. The major protein component of the β-amyloid plaque is Aβ.
Increased accumulation of Aβ has been postulated to significantly contribute to the pathogenesis of AD, and is also associated with various other amyloidoses and neurological disorders, such as Parkinson's disease, Down syndrome, diffuse Lewy body disease, progressive supranuclear palsy, and Hereditary Cerebral Hemorrhage with Amyloidosis-Dutch Type (HCHWA-D), cerebral amyloid angiopathy (CAA), and mild cognitive impairment (MCI). Support for a role for Aβ in AD can be found in Down patients who develop AD-like symptoms and pathology after age 40. Such patients exhibit AD-like amyloid plaques prior to the onset of other AD symptoms, suggesting that increased amyloid accumulation is an initial pathogenic event. Additional evidence implicating accumulation of Aβ peptides in AD comes from the identification of various mutations that result in increased formation of Aβ by cells that account for certain types of inherited AD (familial AD, or FAD). FAD individuals comprise 10% of all AD cases and generally exhibit symptoms of the disease much earlier than sporadic AD patients.
Aβ peptides are derived from processing of an amyloid precursor protein (APP). mRNA generated from the APP gene on chromosome 21 undergoes alternative splicing to yield about 10 possible isoforms, three of which (APP695, 751, and 770 amino acid isoforms) predominate in the brain. APP695 is the shortest of the three isoforms and is produced mainly in neurons. APP751, which contains a Kunitz-protease inhibitor (KPI) domain, and APP770, which contains both the KPI domain and an MRC-OX2 antigen domain, are found mostly in non-neuronal glial cells.
The major APP isoforms are single-transmembrane proteins, composed of an extracellular amino-terminal domain (approximately 590–680 amino acids) and a cytoplasmic tail containing intracellular trafficking signals (approximately 55 amino acids). Within APP, the Aβ peptide sequence is located partially on the extracellular side of the membrane and extends partially into the transmembrane region. APP isoforms 695, 751, and 770 share the same Aβ, transmembrane, and intracellular domains.
APP is trafficked through the constitutive secretory pathway, where it undergoes post-translational processing, including cleavage via two pathways: an amyloidogenic pathway and a non-amyloidogenic pathway. In the non-amyloidogenic pathway, APP is cleaved by α-secretase within the Aβ domain, releasing a large soluble N-terminal fragment (sAPPα) for secretion and a non-amyloidogenic C-terminal fragment (C83). Because cleavage occurs within the Aβ domain, α-secretase cleavage in the non-amyloidogenic pathway precludes Aβ formation. The C-terminal fragment of APP generated by α-secretase cleavage (C83) is subsequently cleaved by γ-secretase within the predicted transmembrane domain to generate a non-amyloidogenic peptide fragment termed p3 (22–24 residues).
In the amyloidogenic pathway, APP is cleaved by β-secretase (BACE1 or BACE2 enzymes) at the beginning of the Aβ domain that defines the amino terminus of the Aβ peptide. Cleavage by BACE1 or BACE2 generates a shorter soluble N-terrninus, sAPPβ, as well as an amyloidogenic C-terminal fragment (C99). Alternatively, BACE1 can also cleave APP 10 amino acids after the beginning of the Aβ domain (between amino acid 10 and 11) to generate a longer N-terminal soluble fragment and a shorter C-terminal fragment (C89). Additional cleavage of either C89 or C99 by γ-secretase, a presenilin-dependent enzyme, produces Aβ peptides of various lengths.
The predominant forms of Aβ found in plaques from AD brains are the Aβ42 and Aβ40 species. Aβ42 is the species initially deposited in brain plaques, and is highly prone to aggregation in vitro. Therefore, the Aβ42 species of amyloid peptide, in particular, may be a viable target in the development of therapeutics for the treatment of disease or disorders characterized by Aβ accumulation.
Currently, there is no cure or effective treatment for AD, and the few approved drugs, including Aricept, Exelon, Cognex and Reminyl, are palliative at best. Based on the correlation between Aβ accumulation, neuronal loss and AD, modulating Aβ levels, such as reducing levels of pathogenic Aβ species, represents a viable way to decrease plaque formation and minimize neuronal cell death. Thus, there exists a medical need for compounds that modulate levels of Aβ. Indeed, such compounds would be useful for the treatment of neurodegenerative disorders, such as AD.